FIG. 1 illustrates a portion of a UMTS wireless communication network. As shown, user equipment (UE) wirelessly communicates with a Node-B serving the communication needs of a geographic area (often referred to as a cell or collection of cells). The UE may be a mobile phone, wireless equipped PDA, wireless equipped laptop, etc. The UE may also be referred to as a mobile station or mobile unit, and the Node-B is often referred to as a base station. Communication from a Node-B to a UE is referred to as downlink or forward link communication, and communication from a UE to a Node-B is referred to as uplink or reverse link communication. In the uplink, various communication channels may exist.
Node-Bs communicate with a radio network controller (RNC), which may configure the UE and the Node-B for communication over enhanced dedicated channels (EDCHs). For example, the RNC may configure an enhanced transport format combination set ETFCS, which may be used by the UE and the Node-B in the uplink direction. The ETFCS may include a plurality of enhanced transport format combinations ETFCs, which may be used for communication between a UE and a Node-B. An ETFC is a selected combination of currently valid enhanced transport formats ETFs, which may be used for transmitting data over an EDCH. An enhanced transport format ETF specifies a data rate to be used for a subsequent transmission over a data portion of the EDCH.
UMTS Radio Access Networks RANs (e.g., Node-B's, RNCs, etc.) use two scheduling methods for transmission over enhanced dedicated channels (EDCH); Node-B scheduling (e.g., MAC-e signaling transmission scheduling), and non-scheduled transmission.
A Node-B scheduler allocates a specific (e.g., a maximum) amount of uplink resources, that a UE may use in the uplink direction, for example, based on Quality of Service related information (e.g., logical channel priority for each logical channel) and scheduling information (e.g., UE buffer capacity, a rate request bit setting, etc.) from the UE. This amount may be sent in a resource indication (or scheduling grant). This indication equates to a traffic-to-pilot ratio, or more specifically, in UMTS to the power ratio between E-DPDCH (the E-DCH dedicated physical data channel) and DPCCH (the dedicated physical control channel, which is always active in uplink).
UMTS-RANs include two types of scheduling grants, an enhanced absolute grant and an enhanced relative grant. An absolute grant is sent to the UE on the enhanced absolute grant channel (EAGCH) providing, for example, the scheduling grant for the UE to be used in future transmissions. The UE may then use this scheduling for transmitting data in the uplink direction by selecting an ETFC, which may consume less resource than given by the scheduling grant. The mapping between scheduling grant and ETFC is provided by the RNC to UE and Node-B.
A relative grant (or update) is sent to the UE on the Enhanced Relative Grant Channel (ERGCH) and serves as a complement to the absolute grant. A relative grant may adjust (e.g., increase or decrease) the scheduling grant, and hence the selected enhanced transport format combination (ETFC) provided in an absolute grant, and may have one of three values, “Up”, “Down”, and “Hold”. A relative grant may be generated by the Node-B, for example, in response to an “Up” rate request bit received from the UE over an enhanced dedicated physical control channel (EDPCCH). A rate request bit (e.g., a happy bit (HP)), which may indicate whether the UE is satisfied with the current parameters (e.g., the maximum ETFC) provided by a previous absolute grant or relative grant.
If the UE has power available to transmit data at a higher ETFC and the total amount of data in the transmit buffer would require a greater number of Transmission Time Intervals (TTIs) than currently allotted (e.g., via the previous scheduling grant), the UE may transmit an “Up” rate request bit. The Node-B may then transmit a relative grant “Up” over the ERGCH to the UE in response to the received “Up” rate request bit. The relative grant “Up” allows the UE to increase the scheduling grant by a specific amount so that the UE is now allowed to select the next higher ETFC value in the ETFCS.
On the other hand if the Node-B detects the need to reduce the scheduling grant given for a specific UE, it may send a relative grant “Down” over the ERGCH to the UE. The relative grant “Down” will instruct the UE to reduce its scheduling grant by a specific amount, so that the UE is only allowed to select ETFC up to the next lower ETFC value in the ETFCS.
To evaluate the appropriate scheduling grants, the EDCH scheduler at the Node-B uses timely information about the consumed resources for each UE under its control, and sends the scheduling grants timely to the UEs in order for the grants to apply to the correct hybrid ARQ (HARQ) process (e.g., the process of sub-packet transmission well-known in the art). However, unlike HSDPA the users on EDCH are allowed to send asynchronously and hence the arrival of data for a specific HARQ process may be spread over an entire UMTS frame, which is equivalent to 10 msec. FIG. 2 illustrates the problem, which is discussed in more detail below.
According to the 3GPP UMTS standard the transmissions scheduling grants (e.g., relative and absolute grants), acknowledgement (ACK) and non-acknowledgement (NACK) messages start at the beginning of a subframe of 2 msec length, which is time-aligned to the downlink system frame of 10 msec length. As indicated before, time-arrivals of the received data on E-DCH are spread over the duration of one UMTS frame. After some time an ACK or a NACK needs to be sent to the UE in order to trigger the corresponding action for the next transmission on this specific HARQ process. The action may be a retransmission of the same data in case of a NACK or a transmission of new data in case of an ACK. The starting point for the transmission of the control information is well-defined in the standards, giving the Node-B sufficient time Tresponse for generating the response, accordingly. In the example of FIG. 2, the Node-B must generate an ACK/NACK for UE#1 at the start of the second subframe, and an ACK/NACK for UE#2 at the start of the 5th subframe, giving times Tresponse1 and Tresponse2, respectively. If the scheduler wants to send a relative grant (RG), which is based on the received data information for that specific HARQ process, the scheduler needs to be ready at the same time. On the other hand, the scheduler needs to collect the received data information from all users that have sent E-DCH data within one transmission time interval (TTI), because it needs the information of the sum of all users in order to make a correct decision.
Accordingly, there is now the dilemma that on the one hand the scheduler should respond as fast as possible to the received data in order to align the RG to the specific HARQ process but on the other hand the scheduler needs to collect the data for all users in order to get the limit of the total allocated E-DCH resources.